One of the techniques known in the art for depositing films of amorphous or crystalline semiconductors, as well as insulating films, is chemical vapor deposition. In this technique, deposits onto a substrate are produced by heterogeneous gas-solid or gas-liquid chemical reactions at the surface of the substrate. A volatile compound of the element or substance to be deposited is introduced into a reactor and decomposed as by thermal means, or reacted with other gases or vapors, at the surface of the substrate in order to yield non-volatile reaction products which deposit on the substrate surface. Chemical vapor deposition processes are well-known for the deposition of silicon films, and insulators of silicon.
In a particular type of chemical vapor deposition, termed homogeneous chemical vapor deposition (HCVD), a homogeneous reaction is produced by decoupling the temperature of the gas and the substrate. This contrasts with conventional CVD where both the source gases and the substrate are generally hot and are at about the same temperature. By decoupling the temperature of the source gas from the substrate, the substrate can be kept at a much lower temperature. This has advantages in many fabrication processes, and in particular in the fabrication of hydrogenated amorphous silicon.
In a conventional apparatus for HCVD, the source gas is thermally heated to its pyrolyzing temperature, as by using a hot-wall reactor, and is pumped to the vicinity of the substrate or carried thereto by a carrier gas. HCVD relies on the gas phase (homogeneous) decomposition of the source molecules, rather than on surface (heterogenous) decomposition as in standard CVD techniques.
As an example of HCVD, films of silicon can be deposited on a substrate using a source gas such as silane. The silane will be homogeneously decomposed at a high temperature and a low pressure, with a film of silicon being deposited upon the low temperature substrate. The silane is drawn through a furnace-heated reactor containing a pedestal on which the substrate is located. The pedestal is cooled, as by nitrogen flow, to maintain its temperature separate and below that of the gases in the reactor. When the source gas is heated to its pyrolysis temperature, an adequate deposition rate onto the substrate is obtained from the homogeneous decomposition reaction by operating just below the gas phase nucleation threshold. HCVD is described in more detail in the following references:
B. A. Scott et al APPL. PHYS. LETT., 39, 73 (1981)
B. A. Scott et al APPL. PHYS. LETT., 40, 973 (1982)
B. A. Scott et al J. DE PHYSIQUE 42 , C4-635 (1981)
B. A. Meyerson et al J. OF APPL.PHYS., 54 , 1461, March 1983).
Although HCVD is an advantageous process which can be used at low pressures and low temperatures to produce good quality films, this process is not without problems. For example, the major problems associated with HCVD have been the following:
1. low deposition rate,
2. homogeneous nucleation of particulates, and
3. a large depletion of the source gas on the hot reactor walls without the deposition of useful product.
It has been found that conventional HCVD hot-wall reactors do not localize decomposition of the reactants in the vicinity of the substrate, and do not provide steep thermal gradients between the gas phase (hot) reactants and the solid surface (cold) upon which the film is to be grown. If this gradient is not as steep as possible, clusters of particulates will form in the hot gas which either deposit on the walls of the reactor or are swept away as unreacted molecules. This in turn depletes the available supply of film precursor. The extended hotwall reactors also increase the liklihood of gas phase nucleation of particulates. The chemistry of the reactions which occur during the preparation of, for example, amorphous silicon by HCVD are described by B. A. Scott et al, J. APPL. PHYS. 54 (12), page 6853, December 1983 (see paragraph A, page 6855).
A technique for heating reactive source gases in CVD other than by a so-called "hot wall" reactor (which is a furnace generally providing heat along the outside wall of a chamber in which the source gas travels) is the type of CVD termed laser-induced CVD. This technique is described in more detail by R. Bilenchi et al, J. APPL. PHYS. hpl.53, p. 6379, September 1982. In this technique laser light, such as that produced by a CO.sub.2 laser, is directed into a gas in order to heat it so that it will decompose. However, this technique is difficult to scale-up, and often provides an uneven temperature distribution.
In laser-induced CVD, the wavelength of the light has to be matched to a vibrational mode in the gas in order to transfer energy to the gas. This often requires that an additional gas be added to the source gases. For example, SiH.sub.4 (silane) does not absorb CO.sub.2 laser light so in order to absorb the laser light SiF.sub.4 is added. The SiF.sub.4 absorbs the light, and then transfers heat to the silane. Energy is distributed through the vibrational modes of the SiF.sub.4 in a manner in which relaxation will not occur rapidly so that heat can be transferred to the silane for decomposing it. It is also desirable that the additive (such as SiF.sub.4) not decompose to contaminate the decomposition products which are to deposit onto the substrate. Since an additive gas with many vibrational modes is required, the choice of the additive is severely limited. For example, more inert gases such as He, Ne, and Ar cannot be efficiently heated with laser light. These inert gases are preferable to use because they will not decompose to adversely participate in the reaction causing the deposited film. Gases such as nitrogen and hydrogen are also poor additives, since they only have one vibrational degree of freedom and relax very readily to a ground energy state. This means that heat will not be efficiently transferred from the hydrogen or nitrogen to the source gas to be decomposed.
In the choice of the additive to be used to absorb laser light, it is also necessary that the additive gas not have a radiational mode, since the energy pumped in by the laser light would then be lost as reradiated light. Accordingly, it is a primary object of the present invention to provide improved reactor design concepts to eliminate the aforementioned problems and to further enhance the generality of HCVD in preparing both amorphous and crystalline forms of insulating and electronic materials.
It is another object of this invention to provide enhanced deposition rates in HCVD.
It is another object of this invention to provide improved HCVD in which homogenous nucleation of particulates is minimized.
It is another object of this invention to provide improved HCVD in which source gas depletion in the reactor is minimized.
It is another object of this invention to provide improved HCVD in which deposition of film precursors onto reactor walls is minimized.
It is a further object of this invention to provide an improved apparatus and method for HCVD for the deposition of amorphous and crystalline films.
It is a still further object of this invention to provide an improved reactor design and technique for HCVD in which a steep thermal gradient is provided between the hot gas phase reactants and the cold substrate onto which deposition is to occur.
It is a still further object of this invention to provide an improved technique and apparatus for HCVD which is suitable for the deposition of thin films over large areas.
It is another object of this invention to provide improved HCVD techniques for processing multiple wafers in which deposition can occur onto a plurality of substrates.
It is another object of this invention to provide enhanced HCVD having an increased range of pressures which can be utilized during deposition.
It is another object of this invention to provide improved HCVD wherein the decomposition of reactants providing the film constituents occurs close to the substrate.
It is another object of this invention to provide improved HCVD which is suitable for the deposition of both insulating and semiconducting films.
It is a further object of this invention to provide improved HCVD for the deposition of both doped silicon films and insulating films of silicon.
In addition to the problems described above with respect to uneven heat distribution, limited availability of additive gases, and the difficulty of scale-up, lasers thamselves are very inefficient light sources (approximately 10%). Thus, while laser-induced CVD is a "cold-wall" approach, it is not without problems.
Accordingly, it is another object of this invention to provide a technique for HCVD which provides an improved cold-wall reactor design.
It is another object of this invention to utilize simple and reliable components for providing a cold-wall reactor.
It is another object of this invention to provide a technique and apparatus and HCVD in which inert gases can be used to provide heat for transfer to source gases.
It is another object of this invention to provide an apparatus and techniques for HCVD wherein uniform heat distribution is obtained through the simple expedient of passing a carrier gas through a hot pipe prior to the transfer of heat from the heated carrier gas to a source gas which is to be decomposed for deposition onto a substrate.